Tetraacetonitrilolithiumhexafluorophosphate and method for the preparation thereof

ABSTRACT

TETRAACETONIRILOLITHIUMHEXAFLUOROPHOSPHATE, A NEW COMPOUND PREPARED BY REACTING LITHIUM FLUORIDE AND PF5 OR PREVIOUSLY PREPARED LIPF6 WITH EXCESS CH3CN, IS DISCLOSED. TETRAACETIONITRILOLITHIUMHEXAFLUOROPHOSPHATE IS USEFUL FOR THE PRODUCTION OF HIGH PURITY EXCEPTIONALLY ACTIVE LIPF6 WHICH IS ALSO A NEW COMPOSITION OF MATTER. THE PREPARATIONS OF THESE NEW COMPOSITIONS ARE ALSO DISCLOSED.

United States Patent Oifice TETRAACETONITRILOLITIHUMHEXAFLUORO- PHOSPHATE AND METHOD FOR THE PREP- ARATION THEREOF Robert A. Wiesboeck, Atlanta, Ga., assignor to United States Steel Corporation N Drawing. Filed May 29, 1969, Ser. No. 829,111 Int. Cl. C07c 12/28 US. Cl. 260-4653 4 Claims ABSTRACT OF THE DISCLOSURE Tetraacetonitrilolithiumhexafluorophosphate, a new compound prepared by reacting lithium fluoride and PF or previously prepared LiPF with excess CH CN, is disclosed. Tetraacetonitrilolithiumhexafluorophosphate is useful for the production of high purity exceptionally active LiPF which is also a new composition of matter. The preparations of these neW compositions are also disclosed.

BACKGROUND OF THE INVENTION The invention relates to lithium compounds and more particularly to improved lithium fluorophosphates.

The preparation of LiPF is well known. It can be prepared by the action of bromine trifluoride on LiF and an excess of P 0 but the product always contains LiF. When prepared by the action of PR; on LiF in anhydrous HF typical purities are 90-95% LiPF Thus, conventional methods produce only impure LiPF Further, the latter method requires the use of hazardous HF as a solvent and thus is not easily adaptable to commercial use. Further, the product may contain Lil-TF as an impurity. This contains protons which are very detrimental for some uses such as for anhydrous batteries and the like. The purification is complicated due to the hydroscopicity and limited thermal stability of LiPF Dissociation to PF and LiF begins to take place at about 20 C., making purification and even removal of solvents diflicult. Certain industrial applications such as in the electric current producing cell of US. Pat. 3,415,687, require LiPF of highest purity, above about 99%, for best performance. However, conventional methods, as pointed out above, produce at best material of up to 95% MPH. This material is not entirely suitable because the impurities can interfere with storage stability and solubility of the material.

SUMMARY OF THE INVENTION The invention provides tetraacetonitrilohexafluorophosphate, Li(CH CN) PF and improved lithium hexafluorophosphate, LiPF derived therefrom as Well as methods for the preparation of these new compositions.

may be prepared in accordance with the invention by the action of excess CH CN on LiF and PP or by the action of excess CH CN on even impure LiPF at a temperature of about 40 C. to about 80 C., preferably 0-80" C. for Li PF starting material and -10 to C. of LiF and PF 5 starting material. The tetraacetonitrilolithiumhexafluorophosphate can be prepared by several alternate routes beginning with the basic raw materials acetonitrile, phosphorus pentafluoride and lithium fluoride. The order of addition of these basic components is not critical to the success of the overall process, but it is strongly preferred to follow a certain order of addition for most economic and satisfactory operation of the process. Thus it is possible to combine the phosphorus pentafiuoride with an excess of acetonitrile in the absence of any lithium fluoride. This, however, will cause the formation of a precipitate which is probably an adduct between phosphorus pentafluoride and acetonitrile which must then in turn be reacted in slurry form with lithium fluoride in order to cleave the adduct and form the desired compound. A better procedure is to first combine the lithium fluoride with an excess of acetonitrile since the lithium fluoride is more reactive than the acetonitrile toward the PF Combination of lithium fluoride with excess acetonitrile, with subsequent addition of phosphorus pentafluoride, therefore, leads to a smooth and economical process. It is important, but not crucial, to the preferred process, therefore, that the reaction system always contain a stoichiometric amount or less of the phosphorus pentafluoride. The Li(CH CN) PF can be isolated from the excess CH CN by removal of the latter under vacuum. It can be isolated in very pure form of 99% or better by separating the solution thereof from any impurities and cooling the filtrate below about 0 C. under partial vacuum with withdrawal of CH CN which is not chemically bound into the new compound.

Only acetonitrile acts on LiF and PF or LiPF to form the new compound, Li(CH CN) PF The nearest homologue, propionitrile, as is illustrated by an example given below does not form a compound. Further, the CH CN does not react with or dissolve LiF or other common impurities in conventionally prepared LiPF so that the Li(CH CN) PF can be easily separated from the impurities and used to produce pure LiPF Therefore, the action of excess acetonitrile on LiF and PF is unique and provides a useful new compound.

One use for Li(CH CN) PF is the production of improved, high purity LiPF When Li(CH CN) PF is warmed above about 20 C. under a partial vacuum it dissociates into LiPF and CH CN. If the warming and partial vacuum are continued until substantially all CH CN has been evolved and separated, a LiPF of exceptionally high purity and high surface area is obtained. LiPF prepared by this process can be used where the highest purity LiPF heretofore obtainable has not been entirely satisfactory, e.g. for the preparation of the electrolyte solution in organic solvents for use in anhydrous electric cells such as in US. Pat. 3,415,687.

Removal of the CH CN in the solid state produces a highly surface active LiPF which dissolves readily even in solvents in which conventionally prepared LiPF is only slightly soluble such as propylene carbonate, methyl acetate, nitro methane and the like. The purity of the resulting LiPF is above 99%, provided that the starting Li(CH CN) PF is at least 99% pure and the CH CN is completely removed.

The compound Li(CH CN) PF is unique in that acetonitrile solutions of the compound can be heated at C. for three hours without excessive decomposition whereas LiPF decomposes at much lower temperatures of e.g. 30 to 40 C. to LiF and PF It is pointed out, however, that the solubility of Li(CH CN) PF in acetonitrile is strongly temperature dependent. A saturated solution contains 82 g./l00 ml. at 60 C. and 11 g./ ml. at 0 C. Excess CH CN may be removed in any suitable manner but vacuum evaporation at 1-0 C. to 0 C. is preferred.

The pure crystals of Li(CH CN) PF melt at 65 to 66 C. without decomposition. By contrast LiPF exhibits a PF equilibrium pressure of 60 mm. Hg at 65 C. There is no dissociation of Li(CH CN) PF to LiF and P'F until all of the CH CN has been removed. In other words, when Li(CH CN) PF is heated under partial vacuum all of the CH CN is evolved before there is any decomposition of the LiPF Another use for Li(CH CN) PF is as a polymerization catalyst for cyclic ethers or unsaturated hydrocarbons.

The invention is further illustrated by the following examples.

3 EXAMPLE I The preparation of Li(CI-I CN) PF Phosphorus pentafluoride was introduced into a slurry of 23 g. of LiF in 1 liter of anhydrous, freshly distilled acetonitrile while cooling to C. and stirring vigorously.

After approximately 125 g. of PF had been absorbed the gas introduction was terminated and the slurry was warmed to 60 to 70 C., filtered and cooled to 0 C. The precipitate was collected by filtration and dried in vacuum at 0 to 5 C. A total of 82 g. of Li(CH CN) PF melt ing at 65 to 75 C., was obtained.

X-ray diffraction pattern was as follows:

A. Intensity, I A. Intensity,

percent percent EXAMPLE II Stability test of Li(CI-I CN) PF in acetonitrile A solution of 80.0 g. Li(CH CN) PF in 100 ml. anhydrous acetonitrile (freshly distilled from calcium hydride) was heated to 80C. for three hours while excluding moisture by a stream of dry nitrogen. After cooling to ambient temperature and storage overnight, the precipitated crystals were removed by filtration. Concentration of the filtrate to ml. and cooling to 0 produced a second crop of crystals. The combined precipitates were dried in vacuum at 0 yielding 73.5 g. of Li(CH CN) PF (92% recovery).

EXAMPLE III Propionitrile as solvent for LiPF Lithium hexafluorophosphate (20.0 g.), prepared from lithium fluoride and phosphorus pentafluoride, was dissolved in 100 ml. dry acetonitrile at 55 C. The solution was stored at ambient temperature for several days and was then slowly concentrated in partial vacuum. No precipitate formed. An oil separated on cooling to 0 C. which resisted all attempts to induce crystallization by customary methods.

EXAMPLE IV Li(CH CN) PF as polymerization catalyst 4 EXAMPLE v The preparation of lithium hexafluorophosphate A 2-liter stirred autoclave was charged with 82.0 g. lithium fluoride, evacuated and cooled to --78 C. One liter of anhydrous hydrogen fluoride was condensed into the reactor and the mixture was warmed to 25 C. while stirring. After one hour the autoclave was pressurized with phosphorus pentafluoride until a constant pressure of 50 psi. was reached. Excess phosphorus pentafluoride and the solvent was removed the following day by condensation into an evacuated cylinder cooled with liquid nitrogen. The autoclave contained 383 g. of crude lithium hexafluorophosphate (92.1% LiPF Another possibility is to react LiF with PF in the absence of HF, but the reaction takes longer and the product is even more impure. It can, however, be reacted with CH CN to prepare the Li(CH CN) PF Crude lithium hexafluorophosphate (620 g.) prepared as above was added to one liter of anhydrous acetonitrile while stirring. The temperature of the slurry rose to C., and was further increased to C. by external heating. Insoluble material was removed by filtration. The brown solution was decolorized by activated carbon. On cooling to room temperature, large colorless needles precipitated and were collected. A second crop was obtained by cooling the filtrate to 10 C.

Drying of the combined precipitates in vacuum at 0 to 5 C. produced 1130 g. of Li(CH CN) PF The compound melted at 65 to C. Complete removal of the acetonitrile was achieved by warming to 30 C. in an evacuated system with an attached cold trap maintained at 78 C. Yield: 551 g. of 99.7% LiPF It is to be understood that the foregoing working examples are given for the purpose of illustration and that any other processes, order of addition, temperature or the like set forth above may be used, provided that the teachings of this disclosure are followed.

I claim:

1. Tetraacetonitrilolithiumhexafluorophosphate.

2. A method for the preparation of tetraacetonitrilolithiumhexafluorophosphate which comprises reacting excess acetonitrile with lithium fluoride and phosphorus pentafluoride at a temperature of about -40 C. to about 80 C.

3. The method of claim 2 wherein said lithium fluoride and phosphorus pentafluoride are prereacted to form lithiumhexafluorophosphate.

4. The method of claim 2 wherein said reaction temperature is in the range of from about 0 to about 80 C. and tetraacetonitrilolithiumhexafluorophosphate is separated from impurities and then cooled below about 0 C. under partial vacuum to remove the excess acetonitrile.

References Cited UNITED STATES PATENTS 3,475,479 10/1969 Vullo 260-4658 JOSEPH P. BRUST, Primary Examiner US. Cl. X.R. 2350, 89 

